In an AC motor drive device, while a large drive current flows for acceleration at the time of power running of the AC motor, a regenerative current is generated at the time of deceleration. If such regenerative power of the motor is simply consumed by a resistor and discharged as heat, energy use efficiency deteriorates, which is not desirable.
Conventionally, therefore, a power storage system is put between a DC-operated inverter and an AC-operated inverter, in parallel with the inverters. The energy storage system includes a power storage element such as a large-capacity electrolytic capacitor or an electric double layer capacitor, a DC/DC converter provided between the power storage element and a DC bus of a DC-operated converter, and a control circuit that controls the DC/DC converter to perform charge and discharge between the DC bus and the power storage element.
For example, as described in Patent Literatures 1, 2, and 3, in an AC motor drive device having a power storage system mounted therein, at the time of power running, power accumulated in a power storage element is discharged to a DC bus by a DC/DC converter, which is then converted to AC power by an inverter and supplied to an AC motor. On the other hand, at the time of regeneration of the motor, the regenerative power causes a voltage of the DC bus to rise via the inverter. Therefore, the bus voltage is charged and accumulated in the power storage element by the DC/DC converter. Thereafter, power accumulated in the power storage element is discharged to the DC bus by the DC/DC converter, thereby performing power regeneration. Accordingly, in the AC motor drive device having the power storage system mounted therein, leveling of a motor drive current can be realized and the regenerative power can be effectively used.